The present invention relates generally to the detection of geophysical anomalies. More specifically, it relates to a method and apparatus for electromagnetic sounding of geophysical anomalies.
One application for techniques that detect geophysical anomalies is directed at the detection and identification of hydrocarbon traps within the earth, either under land or under water, particularly for oil and natural gas traps. Currently, a number of techniques are available for providing data from which such detection can be made by electromagnetic sounding of part of the Earth. These techniques exploit the electromagnetic-response differences that result from the different electrical properties of hydrocarbon traps. Electromagnetic waves scattered through the Earth in the region of such hydrocarbon traps may generate a response that includes anomalies when compared with the electromagnetic response for a homogeneous Earth. An explanation of specific geophysical characteristics that give rise to such different electrical properties is provided, for example, in U.S. Pat. No. 5,563,513, which is herein incorporated by reference for all purposes.
Techniques for generating and propagating the electromagnetic fields, and for collecting the response signals, generally use a structure such as described in U.S. Pat. No. 3,315,155, which is herein incorporated by reference for all purposes. In such structures, a source for direct current is provided in electrical communication with a conductive loop and a damping resistor through a switch. When current flows through the conductive loop it generates a propagating electromagnetic field in accordance with the Maxwell equations. In such applications, the current is provided with a continuous bipolar waveform.
Typically, current is permitted to flow according to a damped sinusoid or bell-like profile having a duration between about 1 and 50 milliseconds depending on the operating characteristics of the power source and on the conductance of the Earth in the region being examined. With such a current profile, each current step induces eddy currents in the subsurface, which in turn induce magnetic-field changes that are measured by a magnetic-field receiver. The electrical structure, such as resistivity and conductance characteristics, is deduced from the amplitude and shape of the received magnetic fields. Methods for reducing noise may also be incorporated, such as that described in U.S. Pat. No. 4,837,514, which is herein incorporated by reference for all purposes, and in which a method is described for increasing the depth for electromagnetic sounding by simultaneously measuring and accounting for three orthogonal components of the ambient noise and decay of the induced current.
There are limitations to such methods dictated primarily by the simple fact that increased depth for electromagnetic sounding generally requires larger signal strength, which itself requires a larger power supply. The square wave in typical applications is provided with an amplitude between about 50 and 300 amperes. In one study, described in Keller et al., xe2x80x9cMegasource time-domain electromagnetic sounding methods,xe2x80x9d Geophysics, 49, 953 (1984), which is herein incorporated by reference for all purposes, a square wave having a peak-to-peak amplitude of 2000 amperes was provided through a grounded antenna. However, this required including a power supply that generated 1,000,000 watts of power.
There is accordingly a need in the art for a system that allows the depth of investigation to be increased while simultaneously reducing the power of the transmitter power supply.
Thus, embodiments of the invention provide a method and apparatus that allows an increase in the strength of an electromagnetic sounding signal, while simultaneously reducing energy consumption, and thereby also permitting smaller weights and dimensions for the power supply. In one embodiment, a method is provided for detecting geophysical anomalies. Electrical energy is capacitively accumulated with a charging circuit until a breakdown voltage of a switch is reached. Thereafter, the accumulated electrical energy is discharged by passing a discharge current through a transmitter antenna to transmit an electromagnetic pulse into a geophysical region. The discharge current may define a circuit having a magnetic moment up to 109-1011 Am2. Alternatively, the discharge current may define a linear moment up to 108-1010 Am. The geophysical region may comprise a volume of earth, may comprise a volume of water, or may comprise a volume of earth beneath a water body in different embodiments.
The method may also comprise receiving a response electromagnetic signal with a receiver antenna. The response electromagnetic signal is transformed into a response electrical signal, which is then compared with the discharge current. Such a comparison may comprise Fourier transforming time dependencies of the discharge current and of the response electrical signal into a frequency domain. Additionally, the discharge current may be synchronized with the response electrical signal as part of the comparison.
In one embodiment the discharge current comprises a damped sinusoid having a frequency co less than 250xcfx81/xcexc0r2, where xcfx81 is an average resistivity of the geophysical region, xcexc0 is the permeability of space, and r is a separation between the transmitter antenna and the receiver antenna. In another embodiment, the discharge current comprises a damped sinusoid and electrical energy is accumulated and discharged for a plurality of distinct frequencies of the damped sinusoid. In another embodiment, the discharge current comprises a bell-shaped pulse.
In one embodiment, the charging circuit includes a low-voltage power supply, a high-voltage power supply, and a first resistor connected in series with an impulse discharge capacitor. In addition to the transmitter antenna, the discharge circuit may include a second resistor connected in series with the impulse discharge capacitor and connected in parallel with the charging circuit.
In an embodiment, the switch comprises a first electrode in electrical communication with the impulse discharge capacitor and a second electrode in electrical communication with the transmitter antenna, the first and second electrodes being separated by a discharge gap. The first and/or second electrodes may be configured to be moveable with respect to each other.
In an embodiment, the transmitter antenna comprises at least two parallel wires, the ends of which are connected. In another embodiment, the transmitter antenna comprises a partially shielded circuit arrangement. A conductive portion of the transmitter antenna may be housed within a hermetically sealed enclosure and electrically insulated enclosure. In one particular embodiment, the conductive portion of the transmitter antenna may be housed within a flexible hermetically sealed and electrically insulated enclosure with a gas at more than atmospheric pressure. In another particular embodiment, the discharge gap is also housed within the hermetically sealed enclosure with a gas at less than atmospheric pressure. In still a further particular embodiment, the first and second electrodes are housed within a solid tube, within which a gas is flowed from an inlet to an outlet of the tube.
A receiver arrangement may include a receiver antenna configured to detect response electromagnetic signals. A first recording element is connected with the discharge circuit and a second recording element is connected with the receiver antenna. A synchronization system is configured to synchronize signals received by the first and second recording elements. In a further embodiment, the receiver arrangement includes an acoustic receiver configured to detect response acoustic signals.